Generally, in a manual transmission (manual gear-shift transmission), when the driver operates the change-lever for a shift, the force applied by the driver in the operation is transmitted through a gear-shifting device to a shift fork, so that the shift fork sets a corresponding clutch (for example, a synchromesh mechanism) into engagement, establishing a desired speed change ratio for the transmission. Such clutches are arranged over either of the parallel input and output shafts of the transmission, in correspondence to permanently meshing gear trains of various speed change ratios, which gear trains are disposed over these parallel shafts in parallel with one another.
For a smooth gear shifting involving little resistance, such a transmission generally incorporates a frictionally synchronizing mechanism, which synchronizes the rotations of respective gears by a friction generated between the conical faces of the gears. This frictionally synchronizing mechanism comprises a clutch gear having a conical face, a synchronizer ring also having a conical face, and a synchro-hub. The clutch gear is provided on one of the input and output shafts of the transmission while the synchro-hub is provided on the other shaft, on which a synchro-sleeve is provided being slidable by a shift fork in the axial direction of the transmission.
When the driver operates the change-lever for a gear shift, the operational force is transmitted through the gear-shifting device to the shift fork, which shifts the synchro-sleeve. This shift of the synchro-sleeve causes the synchronizer ring to move toward the clutch gear, and the conical face inside the synchronizer ring fits to the conical face outside the clutch gear. As a result, a friction is generated between the synchronizer ring and the clutch gear. This friction acts to synchronize the synchronizer ring and the clutch gear to a same rotational speed, leading the splines provided outside the clutch gear to fit into the splines inside the synchro-sleeve.
By the way, when the synchro-sleeve is about to engage with the clutch gear after the synchronizer ring and the clutch gear have been synchronized, if these splines are not in alignment for a fitting, then the clutch gear rotates with respect to the synchro-sleeve and then engages with it. Because this engagement is established through the rotations of the synchronizer ring and the clutch gear driven by the synchro-sleeve, the synchro-sleeve itself receives from the clutch gear a reaction in the direction opposite to that of the shifting of the synchro-sleeve. This reaction force is referred to generally as “two-step engaging load”.
Because this two-step engaging load is transmitted from the synchro-sleeve through the gear-shifting device to the change-lever, the driver, who operates the change-lever actuating the synchro-sleeve, must provide a relatively large force to prevail against the two-step engaging load for a gear shift. Therefore, for the driver to operate the change-lever comfortably, it is important to minimize the two-step engaging load, which acts to impair the maneuverability of the change-lever.
Not to generate such a two-step engaging load, Japanese Laid-Open Patent Publication No. 2000-240683 discloses a method in which the splines inside the synchro-sleeve are provided with chamfer teeth whose chamfer angle at their tooth roots is different from that at their tooth tops. In this method, when the synchro-sleeve pushes the synchronizer ring with the sides of the tooth tops, which have a relatively large chamfer angle, the force acting in the axial direction of the synchronizer ring is relatively large. As a result, a sufficient friction is generated between the synchro-sleeve and the synchronizer ring, facilitating the synchronization. On the other hand, when the synchro-sleeve pushes the synchronizer ring with the sides of the tooth roots, which have a relatively small chamfer angle, the force acting in the circumferential direction of the synchronizer ring is relatively large, facilitating the engagement of the synchronizer ring with the synchro-sleeve. As a result, the maneuverability of the change-lever is improved.
Also, Subaru technique No. 24 “Study on the prevention of 2-step motion in a gear change of manual transmission” issued in 1997, on pages 33˜38, discloses a method for reducing the two-step engaging load, which is generated in gear shifting. In this method, the engagement of the synchronizer ring with the clutch gear is improved by modifying the shapes of the chamfer teeth of the synchro-sleeve and the clutch gear.
In addition to the above mentioned methods, Japanese Laid-Open Patent Publication No. H06(1994)-316227 discloses a method for improving the maneuverability of the change-lever to achieve pleasant shift operation. In this method, the two-step engaging load is damped by an elastic body provided between the change-lever and the vehicle body supporting the change-lever, or the stiffness of the change-lever itself is reduced, or the force required for the operation of the change-lever is reduced by increasing the leverage of the system that transmits the operational force from the change-lever to the synchro-sleeve.
However, the increase of the leverage of the system transmitting the operational force from the change-lever to the synchro-sleeve also increases the stroke of the change-lever. In this respect, such a change-lever has a space limitation in the interior of the vehicle, and also, there is a limit to the increasing of the leverage from the point of view of the maneuverability of the change-lever. Therefore, there is a need for a manual transmission whose gear-shifting device is equipped with a change-lever that provides a maneuverability improved by a method other than the above mentioned method of increasing the leverage.